It is known in the art relating to two stroke engines, also known as two-cycle engines, that air may be inducted the crankcase to charge the cylinders. The crankcase may be divided into separate chambers, one for each cylinder, and the pumping action of the each piston assists in drawing air into each chamber during an up-stroke and then pressuring the chamber to force air into the cylinder during the down-stroke. Fuel is sprayed into each cylinder during its down-stroke, and lubricating oil is sprayed as a mist into the incoming air.
It is also known in such engines to form the chambers in the cast block by walls or webs which have circular openings concentric with the axis of the crankshaft, and seal discs on the crankshaft which fit within the openings to separate each chamber from the next. The U.S. Pat. No. 4,947,807 to Flaig et al shows an example of such an engine. The patent specifically discloses a V-block engine which uses the same crank pin for two cylinders in opposite banks of the engine. A seal disc on the crank pin and a corresponding wall are provided to maintain separate crankcase chambers.
Heretofore, two ways have been known for manufacturing crankshafts with seal discs on the crank pins. One is to assemble the crankshaft of individual main bearing journals, crank pins, counterweights and seal discs, wherein the seal discs have an eccentric opening slightly smaller than the crank pins, and each seal disc is pressed onto a crank pin with an interference fit prior to assembling the pins to adjacent counterweights. The other way is to forge the crankshaft in one piece and machine it to define the crank pins, counterweights, journals and seal discs. Each method of manufacture has certain advantages and may be selected for a given application.
In the case of forged crankshafts, it is not possible to forge the blank to a shape near that of the finished product in the region of the crank pin due to the seal disc requirement, thus mandating extensive machining to define the crank pin and an integral seal disc. Moreover, due to the mist lubrication method, it is desirable that the roller bearings ride directly on the crank pins and main journals, thereby requiring the crank pins and main journals to be surface hardened to 60 to 62 Rockwell C. Thrust faces on the seal discs and counterweights prevent the roller bearings from moving axially and also guide the connecting rods. Thus the thrust faces must also be hardened to meet performance standards. Accordingly, in addition to the extensive rough machining, the crank pins and journals must be hardened and ground to bearing quality and the thrust faces must be hardened as well. It is desirable to provide the advantages of the forged crankshaft at a reduced manufacturing cost.